Security analysts and cartel sources agree that a key factor in the transformation of underworld rivalries into a full-throttle war has been the cartels’ recruitment of elite soldiers.

The leakage of Mexican special forces into organized crime began in the 1990s when the powerful Gulf cartel recruited a group of ex-Gafe troops to create its own paramilitary enforcement unit, known as Los Zetas.

The scale of the problem remains unclear – not least because the Mexican government has been unwilling to release data, said Guadalupe Correa-Cabrera, an associate professor at George Mason University and author of Zetas Inc.

“It’s an inconvenient issue for the government, so they deny freedom-of-information requests. But what we do know is that special forces helped turn Mexico’s narcos into the paramilitary armed groups we see today.”

According to Mexico’s defence ministry, about 1,383 elite soldiers deserted between 1994 and 2015.

Defectors included members of units that received training in counter-terrorism, counter-intelligence, interrogation and strategy from French, Israeli and US advisers, according to a 2005 FBI intelligence document.

Quick Guide

Mexico's war on drugs

On 10 December 2006, Felipe Calderón launched Mexico’s war on drugs by sending 6,500 troops into his home state of Michoacán, where rival cartels were engaged in tit-for-tat massacres.

Calderón declared war eight days after taking power – a move widely seen as an attempt to boost his own legitimacy after a bitterly contested election victory. Within two months, around 20,000 troops were involved in operations.

The US has donated at least $1.5bn through the Merida Initiative since 2008, while Mexico spent at least $54bn on security and defence between 2007 and 2016. Critics say that this influx of cash has helped create an opaque security industry open to corruption.

But the biggest costs have been human: since 2007, over 250,000 people have been murdered, more than 40,000 reported as disappeared and 26,000 unidentified bodies in morgues across the country. Human rights groups have also detailed a vast rise in human rights abuses including torture, extrajudicial killings and forced disappearances by state security forces.

Peña Nieto claimed to have killed or detained 110 of 122 of his government's most wanted narcos. But his biggest victory – and most embarrassing blunder – was the recapture, escape, another recapture and extradition of Joaquín “El Chapo” Guzmán, leader of the Sinaloa cartel.

Mexico’s decade-long war on drugs would never have been possible without the injection of American cash and military cooperation under the Merida Initiative. The funds have continued to flow despite indisputable evidence of human rights violations.

Under new president Andrés Manuel López Obrador, murder rates are up and a new security force, the Civil Guard, is being deployed onto the streets despite campaign promises to end the drug war.

Improved collaboration between the US and Mexico has resulted in numerous high-profile arrests and drug busts. Officials say 25 of the 37 drug traffickers on Calderón’s most-wanted list have been jailed, extradited to the US or killed, although not all of these actions have been independently corroborated.

The biggest victory – and most embarrassing blunder – under Peña Nieto’s leadership was the recapture, escape and another recapture of Joaquín “El Chapo” Guzmán, leader of the Sinaloa cartel.

While the crackdown and capture of kingpins has won praise from the media and US, it has done little to reduce the violence.

Photograph: Pedro Pardo/AFP

Internal documents from Mexico’s attorney general’s office obtained by the Guardian also confirm accounts from sources in Michoacán that the Templars’ predecessor organization – known as La Familia Michoacana – sent envoys to Guatemala to recruit former special forces soldiers known as Kaibiles.

According to Kate Doyle, senior analyst at the National Security Archive in Washington DC, the US focus on military aid to the region has helped drive the militarization of Mexico’s drug conflict.

“That US military training and intelligence techniques ended up in the wrong hands is far from unusual. Its lethal spillage into the contemporary criminal context is one of the legacies of US security policy in Latin America,” she said.

Rarely, however, has a soldier-turned-narco gone on the record.

Between 2006 and 2017, the Mexican military received $2.7bn in security assistance from the US. Photograph: Anadolu Agency/Getty Images

As he led the way up a steep path to a sniper’s nest of volcanic stone and brush, Delfino said he had his own reasons for speaking to a reporter. “We want the world to understand what we’re doing out here: protecting the communities against the enemies that come to rape and pillage.”

The military crackdown was eventually extended across the country, but its initial targets were the cartels of Calderón’s home state: La Familia Michoacana and its offspring, the Knights Templar.

For a time, it seemed that the strategy was working. When the Templars finally collapsed under the joint pressure of federal forces and an armed “self-defense” movement in 2013, the government claimed victory.

But for every fallen kingpin, there was a host of would-be successors: organized crime in Michoacán shattered into a patchwork of warring fiefdoms. And though now reduced to a fraction of their former strength and cut off from vital trafficking routes, the Templars are still in the thick of it.

Their current enemies are a group of former allies, to whom Delfino refers contemptuously as “locusts”.

Up on the hill, he eyed the enemy positions through his scope. Locusts surrounded his position on three sides and had attempted to overrun the Templars several times in recent months.

But Delfino was dismissive of his counterparts, who he said were untrained boys sent into battle doped up on crystal meth.

“The difference between them and us is that we always take precise shots,” he said.

Most of Delfino’s own fighters were barely adults, but he still claimed to abide by the principles of his military training. “The strict rules, the way they prepared me psychologically, morally and practically – that stays with you forever,” he said.

Delfino’s training began not long after he joined the army in 1996. After selection for the Gafe, he underwent months of instruction, including courses in jungle survival and amphibious combat.

After specializing as a sniper, he was deployed in counterinsurgency operations in the southern state of Chiapas, where the indigenous Zapatista guerrillas had risen up in 1994.

‘We want the world to understand what we’re doing out here: protecting the communities against the enemies that come to rape and pillage,’ Delfino said. Photograph: Falko Ernst

Later, he was moved to the Pacific port city of Lázaro Cárdenas, where he became involved for the first time in counter-narcotics: his unit was tasked to chase speedboats bringing cocaine from South America.

But off-duty, Delfino and his fellow soldiers came into contact with narco bosses in local nightclubs. Before long, he was receiving expensive bottles of Scotch – and then job offers.

“Technically it’s not correct to take somebody’s life,” said Delfino. But then he reached for a biblical justification: “Not a leaf moves without God’s permission.”

At the height of the cartel’s power, nothing in Michoacán moved without the cartel’s permission. It monopolized crime, but it also penetrated ordinary life, using the threat of lethal violence to arbitrate anything from land disputes to marital conflicts.

That soft power was fused with strategic sophistication, thanks to the influx of former soldiers, said Correa-Cabrera. “Their rapid expansion, the way they controlled territories, used communications and armament – they were now doing it like the army,” she said.

The involvement of veterans has enabled cartel combatants across the country to organize tactical responses to the deployment of troops and paramilitary federal police. More recently, the CJNG has become notorious for a string of ambushes that have killed dozens of federal officers.

In Tierra Caliente, such head-on confrontations have given way to a constant state of low-intensity warfare. In 2017, 1,510 murders were registered in Michoacán, a state of 4.5 million inhabitants.

Delfino’s role in the bloodletting is no secret to his former brothers in arms. He remains in touch with soldiers on active duty, and even meets up to reminisce when security conditions allow, he said.

“We like each other, and they respect my decision,” he said, “but if they learn that I’m out here doing something which doesn’t square with our values – if I mess with innocent people – they will come for me. From them, there’s no hiding.”